CMS-PAS-EXO-18-008

CMS-PAS-EXO-18-008
Search for an $L_{\mu}-L_{\tau}$ gauge boson using $\mathrm{Z}\rightarrow4\mu$ events in pp collisions at $\sqrt{s}= $ 13 TeV
CMS Collaboration
June 2018

Abstract: A search for a narrow Z' gauge boson with a mass between 5 and 70 GeV resulting from an $L_{\mu}-L_{\tau}$ U(1)' symmetry is reported. Theories that predict such a particle have been proposed as an explanation of various experimental anomalies including the lack of a dark matter signal in direct detection experiments, tension in the measurement of the anomalous magnetic moment of the muon, and reports of possible lepton flavour universality violation in B-meson decays. The data sample consists of proton-proton collisions at $\sqrt{s}= $ 13 TeV collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 77.3 fb$^{-1}$. Events containing four muons with an invariant mass near the Z boson mass are analyzed, and the selection is further optimized to be sensitive to the Z' signal that may be present in $\mathrm{Z}\rightarrow4\mu$ decays. The observations are consistent with the standard model predictions. Upper limits of $ 10^{-8} $-$ 10^{-7} $ at 95% confidence level are set on the branching fraction $\mathcal{B}(\mathrm{Z} \rightarrow \mathrm{Z}^{\prime} \mu \mu) \times \mathcal{B}(\mathrm{Z}^{\prime} \rightarrow\mu \mu)$, which excludes a Z' boson coupling strength to muons above 0.004-0.3, depending on the Z' mass. These are the first dedicated limits on the $L_{\mu}-L_{\tau}$ model at the LHC.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 792 (2019) 345. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order Feynman diagrams for the signal process (left) and the dominant background process (right).
png pdf	Figure 1-a: Leading-order Feynman diagrams for the signal process (left) and the dominant background process (right).
png pdf	Figure 1-b: Leading-order Feynman diagrams for the signal process (left) and the dominant background process (right).
png pdf	Figure 2: Distribution of the reconstructed four-muon invariant mass and a comparison to the predicted $ {{\mathrm {q}} {\overline {\mathrm {q}}}} / {\mathrm {g}} {\mathrm {g}} \to 4\mu $ background. Different Z' signal hypotheses are also shown.
png pdf	Figure 3: Distributions of the reconstructed $m({\mathrm {Z}}_1)$ and $m({\mathrm {Z}}_2)$ observables and a comparison to the predicted $ {{\mathrm {q}} {\overline {\mathrm {q}}}} / {\mathrm {g}} {\mathrm {g}} \to 4\mu $ background. Different Z' signal hypotheses are also shown.
png pdf	Figure 3-a: Distributions of the reconstructed $m({\mathrm {Z}}_1)$ and $m({\mathrm {Z}}_2)$ observables and a comparison to the predicted $ {{\mathrm {q}} {\overline {\mathrm {q}}}} / {\mathrm {g}} {\mathrm {g}} \to 4\mu $ background. Different Z' signal hypotheses are also shown.
png pdf	Figure 3-b: Distributions of the reconstructed $m({\mathrm {Z}}_1)$ and $m({\mathrm {Z}}_2)$ observables and a comparison to the predicted $ {{\mathrm {q}} {\overline {\mathrm {q}}}} / {\mathrm {g}} {\mathrm {g}} \to 4\mu $ background. Different Z' signal hypotheses are also shown.
png pdf	Figure 4: Expected and observed 95% CL limits on the product of Z' production cross section and branching fraction (left y-axis) and $\mathcal {B}({\mathrm {Z}}\rightarrow {\mathrm {Z}}^{\prime} \mu \mu) \times \mathcal {B}({\mathrm {Z}}^{\prime} \rightarrow \mu \mu)$ (right y-axis).
png pdf	Figure 5: Left: expected and observed 95% CL limits on the gauge coupling strength as a function of $m({\mathrm {Z}'})$. Right: comparison with other experiments sensitive to the same parameter space. These constraints are adapted from Ref. [8].
png pdf	Figure 5-a: Left: expected and observed 95% CL limits on the gauge coupling strength as a function of $m({\mathrm {Z}'})$. Right: comparison with other experiments sensitive to the same parameter space. These constraints are adapted from Ref. [8].
png pdf	Figure 5-b: Left: expected and observed 95% CL limits on the gauge coupling strength as a function of $m({\mathrm {Z}'})$. Right: comparison with other experiments sensitive to the same parameter space. These constraints are adapted from Ref. [8].

Tables

png pdf

Table 1:
The number of expected background and signal events and the number of observed candidate events after the full selection with 80 $ < {m_{4 {\mu}}} < $ 100 GeV. The signal and $ {{\mathrm {q}} {\overline {\mathrm {q}}}} / {\mathrm {g}} {\mathrm {g}} \to 4\mu $ background rates are both estimated from simulation. The uncertainties in the signal predictions are purely systematic uncertainties, while in the background predictions the uncertainties also include the statistical uncertainty. Also shown are the number of expected background and signal events and the number of observed candidate events in the relevant mass window for each $m({\mathrm {Z}'})$ hypothesis.

Summary

The first dedicated search at the LHC for a $\mathrm{Z'}$ gauge boson resulting from an $L_{\mu}-L_{\tau}$ U(1)^{\prime} symmetry has been presented. The search places strong constraints on theories that attempt to explain various experimental anomalies including the lack of a dark matter signal in direct detection experiments, tension in the measurement of the anomalous magnetic moment of the muon, and reports of possible lepton flavour universality violation on B-meson decays. The search considers events containing four muons with an invariant mass near the Z boson mass and the selection is further optimized to be sensitive to the Z' signal that may be present in Z$\rightarrow4\mu$ decays. The observations are consistent with the standard model predictions. Upper limits of $10^{-8}$-$10^{-7}$ at 95% confidence level are set on the branching fraction $\mathcal{B}(\mathrm{Z} \rightarrow$Z$^{\prime} \mu \mu) \times \mathcal{B}($Z$^{\prime} \rightarrow\mu \mu)$, which excludes a Z' boson coupling strength to muons above 0.004-0.3, depending on the Z' mass.

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